A team led by Dinglin Jiang at the National Institutes of Natural Sciences in Okazaki (Japan) reports in the journal Angewandte Chemie on the synthesis of co-operative porous frameworks based on aza-fused π(pi)-conjugated microporous polymers (CMPs) for use in supercapacitors. (Aza compounds replace a carbon atom with a nitrogen atom; π-conjugated compounds have alternating single and multiple bonds in their structure.) Aza-CMPs exhibit large capacitance, high power and energy densities (approaching those of current-generation Li-ion batteries), and enable repetitive energy storage and power supply with an excellent cycle life.

Supercapacitors work on a different charge-storage principle than rechargeable batteries, and consist of electrochemical double layers on electrodes, which are wetted by an electrolyte. When a voltage is applied, ions of opposite charge collect on both electrodes to form wafer-thin zones of immobilized charge carriers. In contrast to a battery, there is only a shift of charge; no chemical transformation occurs. Various materials are suitable for supercapacitors, but an ideal material has yet to be found.

Special microporous, framework-like, organic polymers are materials of interest in this area. Their double bonds are arranged in such a way that some of their electrons can move freely over extended regions of the framework as an “electron cloud”. Such materials are thus conducting. A large inner surface area is important for the formation of electrostatic charge-separation layers in the pores.

π-Conjugated microporous polymers (CMPs) are a class
of porous frameworks consisting of an extended π-conjugated system and inherent nanopores.
As high surface-area porous materials, CMPs emerge as a new medium for gas adsorption and have been developed as a new type of nanoreactors and heterogeneous catalysts upon the integration of catalytic sites into the skeletons. The extended π-conjugated system endows CMPs with noteworthy light-emitting properties and allows the construction of light-harvesting antennae that trigger efficient, rapid, and vectorial energy funneling from the skeleton to entrapped acceptors.

From a synthetic point of view, CMPs are unique because they
allow the elaborate control of both skeletons and pores. In
this context, a promising way to the exploration of CMPs is to
combine the structural advantages of a π-conjugated system
and inherent pores. Herein, we report the synthesis of such
co-operative porous frameworks based on aza-fused CMPs and highlight their functions in
supercapacitive energy storage and electric power supply.

Aza-CMPs comprise four features: 1) fused CMP frameworks that are conductive, 2) aza units in the skeletons that enable dipolar interaction with electrolyte cations and accumulate protons on the walls of pores, 3) inherent micropores with optimized size that allows quick ion motion during charge–discharge processes, and 4) high surface areas provide large interfaces for the formation of electrostatic charge separation layers in the pores. Ultimately, these structural features work co-operatively, leading to exceptional energy
storage and power supply capacities.

—Kou et al.

Jiang and his team synthesized a nitrogen-containing framework with a pore size optimal for allowing ions to flow in and out rapidly—a requirement for rapid charging and discharging. The nitrogen centers interact with the electrolyte ions, thus favoring the accumulation of charge and the movement of ions.

Supercapacitive performance was tested with galvanostatic charge–discharge cycling experiments. A general tendency is that when the current density
increased, the charge and discharge times were significantly
shortened. Aza-CMP@350—one of five variants of the material—can be operated at a high current density of 10 Ag-1 to allow power supply at high rates.

The capacitance is even greater than the best values for supercapacitors (720 F g-1) with a redox-active ruthenium oxide electrode.

Energy and power densities. The
Ragone plot reveals that Aza-CMPs exhibit the maximum
energy and power densities of 53 Wh kg-1
and 2.25 kW kg-1 respectively. The
energy density is higher than those of nanostructured porous
carbon materials and reaches the regime of batteries such as
Pb-acid, NiCd, and lithium ion battery (10–150 Wh kg-1). The powder density of Aza-CMPs is more than one order of magnitude higher than those of batteries (< 0.3 kW kg-1).

Stability. Aza-CMP@350 exhibited excellent performance stability without loss in capacitance (397 F g-1) after 10,000 charge–discharge cycles at a
current density of 5 Ag-1. AzaCMP@400 also exhibited a highly stable performance without
any deterioration.

The fused skeleton, dense aza units, and
well-defined micropores work co-operatively and facilitate
electrostatic charge-separation layer formation. Consequently, Aza-CMPs exhibit large capacitance, high energy
and power densities, and enable repetitive energy storage and
power supply with an excellent cycle life. These remarkable
results reported herein demonstrate the enormous potential
of π-conjugated microporous polymers as high-energy storage
devices.

—Kou et al.

This work was supported by the Japan Science and Technology Agency (JST).

pretty impressive, still I am unconvinced that supercap will compete with batteries except in applications where you want a very high number of cycles and a very high rate of charge/discharge like for regenerative braking

Can we please stop propagating the myth that Li batteries can't handle high rates of regen? See how fast a Tesla slows down without using the brakes, and then remember that other cells have even higher C rates than the ones Tesla uses.

Well a Tesla has a huge battery 53 Kwh- a Leaf has less than half 24 Kwh and a Volt less than a third with 16 Kwh and obviously if you are rocking a huge battery your C rate per Kg means less than if you were looking at 2-4 Kwh in a hybrid - but I don't see this as yet something that would be used in BEVs but very useful in Hybrids, scooters, electric bikes.

I don't think there can be any doubt that Li batteries can handle huge regen. I've seen a couple of discussions with F1 principals where they said that the KERS systems over there are using A123 Li phosphates and jamming 20kW/kg into and out of those batteries. That is a phenomenal power rate!

Of course, I've never seen ANYONE admit whether those batteries last a single race of an entire season with that kind of abuse so that is a big unknown :-)

It will be hard for supercaps to compete with batteries, but....Think about this: A supercap could take a 30 second recharge and not even blink. So if you could get a 150 mile range supercap "battery", then would it really be so painful to have to recharge more often?

Of course, I'm assuming there is some way to supply electricity at a rate of 4.2MW! (35kWh*3600seconds/30seconds). LOL That is one hell of a lot of juice flowing...might even make a great bomb if something goes wrong.

Probably be more realistic to settle for a 3 minute recharge at 700kW rate. Safer and a lot more reasonable. Once you've pulled over to fill up, what difference does a couple of minutes make.

Also, think about 10,000 cycles for the supercaps. A 150 mile range means you're getting 1.5million miles out of the pack!

maybe, you can arguee also that supercap are not affected by cold temperature, but in turn their self discharge is pretty bad so doesn't really make it a good candidate for a car. As for fast recharging titanate batteries like the new Toshiba are very good in that regard. Cycling is a point but beyond 2000 cycles it is no longer an advantage for a car applications.

E-storage units will eventually be classified relevant to specific technologies used. The line between solid states ultra caps and solid states batteries is getting thinner and so will their performances.

Yeah, you're right about the self discharge problem for the supercaps...forgot that issue.

And those Toshiba batteries are really good at cycling and great power. But their energy density is only about 100Wh/kg if memeory serves. But then, they can use almost 100% of that charge without really damaging the cycle life.

Sigh, tradeoffs, tradeoffs. The real breakthrough will come when we can stop making so many tradeoffs. :-)

Future generation will find current batteries and ultra cap very primitive. Many new technologies will be used to advance e-storage units performance. Future solar energy and direct heat converters will also make e-energy abundant, clean and relatively cheap.

Our city has stopped buying diesel city buses. All future purchases will be for hybrids and pure electric buses. One bad news, they want to bring back those trolleys with overhead cables on selected streets. That could turn the clock backward 100+ years.

I've noticed with many reports on GreenCarCongress and elsewhere that you need to be careful about the quoted figure of Wh/kg. Researchers sometimes calculate this on the basis of the electrolyte, as this is easier (see e.g. the optimistic report some time ago of 83 Wh/kg carbon nanotube supercapacitors). The figure of Wh/kg for an actual device is then often around ten times lower.